A comparison of plaster casting and pressure casting of sanitaryware with particular reference to clay properties

Abstract

Sanitaryware Body slips have been prepared using a range of ball clays and china clays, and the pressure casting and plaster casting properties of these slips were examined as a function of their state of deflocculation. Some properties of the resulting cast pieces were also investigated. Pressure casting was carried out at two pressures, 13.3 and 40 bar and at the higher pressure, slip temperatures of 20 and 40°C were used. It was confirmed that casting times can be substantially reduced by pressure casting compared with plaster casting. The ratio of pressure casting time to plaster casting time is independant of the ball clay or china clay type used in the body, confirming that the parameters controlling casting times are the same for both pressure and plaster casting. It was also shown that while plaster casting times are significantly influenced by slip thixotropy, pressure casting times are almost independant of this parameter. Moisture contents of pressure cast pieces were lower than those of plaster cast pieces by up to 3 wt% and there was little variation with slip thixotropy. As a result of the decreased moisture contents, drying shrinkages of pressure cast pieces were reduced. Also as a result of their lower moisture contents, the stiffness of pressure cast pieces was equivalent to or higher than that of plaster cast pieces that been dried for two or three hours, thus allowing the pieces to be demoulded immediately after forming.